Abstract: Phenylphosphonic dichloride and 1,10-diaminodecane monomer were used as raw materials to prepare phenylphosphonic diaminodecane by one-step condensation method. Then, vanillin and its derivatives containing monaldehyde, dialdehyde and trialdehyde group were introduced to prepare vanillin-based linear phosphoramide (V1), linear polyphosphoramide (V2) and branched polyphosphoramide (V3) containing Schiff base via condensation reaction. The structures of these three vanillin-based phosphoramide fire retardants were determined by Fourier transform infrared spectroscopy, nuclear magnetic resonance, gel permeation chromatography and X-ray photoelectron spectroscopy, and their thermal stability, thermal degradation behavior and char-forming ability were studied by thermogravimetric analysis and differential scanning calorimeter. The results show that the molecular structure and the content of phosphorus and nitrogen can both affect the thermal stability and carbonization of fire retardants simultaneously. For linear fire retardants V1 and V2, their molecular weight has a great influence on the thermal stability. When molecular weight increases, more C―C chains tend to decompose, so the thermal stability of V2 is lower than that of V1. The branched fire retardant V3 with a high degree of crosslinking has better carbonization properties than linear fire retardants. In addition, due to the acidolysis effect of Schiff base, three kinds of vanillin-based phosphoramide fire retardants can be completely degraded in acetic acid aqueous solution within 4, 12 h and 36 h, respectively.